Bandwidth demands on premises networks continue to rise, driven by more users and bandwidth-hungry applications. The latter include increasingly sophisticated Internet content and interactivity approaches to information exchange such as embedding applets, full-motion video, and sound effects.

This rapid growth and increasing data requirements has caused local area network (LAN) data rates to nearly double each year during the past decade. In 1995, typical data rates to the desktop were 10 megabits per second (Mbps), with 100 Mbps in the backbone. Five years later, those rates have jumped to 100 Mbps to the desktop and 1,000 Mbps - 1 gigabit - in the backbone, and the numbers will leap another order of magnitude by 2005.

This phenomenal growth has made optical fiber a necessity in premises cabling infrastructures. Once confined to backbones, optical fiber is now common in the riser and horizontal, as fiber-to-the-desk designs no longer are the novelties they seemed to be only a few years ago.

Yet this increased use of optical fiber in premises networks has brought confusion: With a variety of fiber types to select from, which optical fiber is best for specific segments of the network? This article will:

- Briefly discuss the advantages of fiber in premises networks;
- Provide a tutorial on the various fiber types available, including light sources used and bandwidth capabilities;
Offer a "Fiber Selection Guide" for inter-building, intra-building, horizontal, and centralized cabling; and
- Briefly describe next generation fibers.

The advantages of fiber in premises cabling

By now, the advantages of optical fiber are well known. With its extremely high bandwidth and very low attenuation (signal loss over distance), optical fiber will accommodate all current and envisioned network protocols without continual recabling.

Because fiber is dielectric, it eliminates most of the concern over factors affecting link performance. It is immune to cross talk, particularly near-end cross talk, or NEXT, electromagnetic interference (EMI), radio frequency interference (RFI), impedance mismatches, transmission frequency variability, and ground loops. Optical fiber's virtually error-free transmission over long distances results in few outages and little downtime, eliminating the need for cumbersome error detection and correction protocols.

Now, the popularity and rapid acceptance of the Gigabit Ethernet (GbE) Standard has given rise to new concerns about installed CAT 5 cabling, which may not always comply with the new demands of GbE. Full duplex transmission on all four copper pairs produces a critical new parameter, far-end crosstalk (FEXT). Old parameters - near-end cross talk (NEXT), return loss, power sum attenuation-to-crosstalk ratio (ACR), delay skew - all become crucial with GbE. Installed CAT 5 cable may not comply with these new or more critical requirements. There also are concerns over noncompliant connecting hardware and poor interoperability among products from different vendors.

As data rates continue to rise, using optical fiber will provide greater network reliability and superior performance. These factors have made fiber the medium of choice in premises backbones, and have extended fiber's reach into the riser and horizontal segments of networks. Therefore, network managers and designers should be aware of what types of fibers are available, and which are appropriate for the specific segments of networks.

Optical fibers exist in two primary types: single-mode and multimode. Both types have the same outer diameter, 125 µm, about the size of a human hair. Multimode fiber, however, has a much larger core - the light-carrying region of the fiber - either 50 µm or 62.5 µm, as compared with 7 to 10 µm for single-mode fiber.

Information is carried through single-mode fiber on a single mode, or ray, of light. Due to the multimode fiber's larger core size, hundreds of modes can propagate through it at the same time. Somewhat counter-intuitively, single-mode fiber offers much higher bandwidth and lower signal attenuation than multimode fiber. This is because using a single-mode fiber eliminates the problem of modal dispersion. Modal dispersion is the spreading of light pulses as modes travel paths of different lengths within a fiber's core. The modes arrive at a receiver at different times, discrete pulses merge, and bit errors occur. Therefore, modal dispersion is the bandwidth-limiting factor in multimode fiber.

[illustration: modal dispersion]

Single-mode fiber is perfect for communication over long distances, because it can transmit signals quite far before they require amplification. In telephony applications, where single-mode is used extensively, amplifiers or repeaters are typically placed at intervals of 50 miles or more. (As a point of comparison, copper-based systems require repeaters approximately every mile.)

For short-link-length premises network applications, single-mode fiber may not be necessary. However, for campus backbone links, a hybrid singlemode /multimode fiber cable should be considered. The single-mode fiber can remain dark, in readiness for a possible future upgrade.

Multimode fiber is preferred for the majority of premises applications. Its bandwidth is more than sufficient to support most LAN applications. Multimode fiber is the cost-effective choice as well: Multimode fiber is also easier and less expensive to terminate than single-mode fiber because it is easier to couple light into its larger core area. Also, multimode fiber-based systems employ inexpensive light-emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs) as light sources, whereas single-mode fiber systems can require more costly lasers, due to packaging and tighter alignment tolerances.

Multimode fibers are available in two core sizes: 50 µm and 62.5 µm. The latter is the fiber distributed data interface (FDDI) standard fiber and has been widely adopted in North America since its introduction in the mid-1980s. However, 50 µm fiber is becoming increasingly important in premises cabling because it offers higher bandwidth than 62.5 does at the 850 nm wavelength. Standard 50 µm fiber has triple the bandwidth of standard 62.5 µm fiber at the shorter wavelength: 500 MHz/km vs. 160 MHz/km. The short wavelength is important because new low-cost 850 nm lasers, such as VCSELs, have been developed and quickly adopted for network applications. By using 50 µm fiber and inexpensive laser transmitters, network designers can achieve longer link lengths and higher data rates than what is possible with 62.5 µm fiber.

Selecting fiber for premises cabling

Multimode fiber coupled to LEDs will operate at distances up to 2,000 meters at 155 Mbps or less. Much higher data rates, up to 1 gigabit per second (Gbps), currently can be achieved for the following shorter-distance applications:

- Intrabuilding, or riser (

- Horizontal (

- Centralized (

The following "Fiber Selection Guide" indicates which fiber types are appropriate for network protocols by link length. Wherever there is a choice, a single-mode solution is the most expensive option. Also, keep in mind that new laser-optimized multimode fibers - the topic of the last section of this article - are being developed or have already been developed to offer improved performance to support higher bit rates and even longer link lengths. Please keep this in mind while looking at the selection guide. This guide is a reference tool based on standards and does not represent all multimode fibers available.
To use the guide, determine your distance and needs, look for your application, and locate the appropriate fiber type. To ascertain specific link length recommendations, refer to the following:

- For 155 Mbps, 622 Mbps and 2.4 Gbps ATM - ATM Forum PHY documents.

- For 10 BASE, and Fast and Gigabit Ethernet - IEEE 802.3 documents.

- For all protocols - TIA 568A, 569 and TSB-72 standards for infrastructure, as well as TIA 492 and IEC standards for fiber recommendations.

As data rates continue to climb, the key to improved performance in premises networks is increased bandwidth over longer link lengths. Low-cost lasers and laser-optimized multimode fibers are providing the means to transmit data at gigabit speeds over distances required by LANs very cost effectively. Moreover, soon multimode fiber solutions will be available to support 10 Gbps serial transmission.

VCSELs have been instrumental in achieving very high data rates in premises applications. These inexpensive laser sources operate at 850 nm, making them ideal for use with multimode. Single-mode fiber is not designed to operate at 850 nm. The combination of laser-optimized 50µm multimode fiber and VCSELs represents the lowest-complexity, lowest-cost option for high-speed premises networks as they move steadily into multi-gigabit data rates.

The near future will require even higher bandwidths and greater speeds from laser-optimized multimode fiber. The next generation 50 µm multimode fiber is expected to support 10 Gbps over 300m, as well as slower speeds over much longer distances. Standards bodies are hurrying to keep up with this pace of change. Gigabit Ethernet (IEEE 802.3z) was standardized in June 1998, and efforts already are underway to complete a 10 Gigabit Ethernet standard (IEEE 802.3ae).

Next generation multimode fiber will support legacy LAN applications while providing the least system complexity at the lowest cost. Moreover, these laser-optimized fibers will provide a migration path to 10 Gbps using 850 nm transceiver technology. These developments will continue to make multimode fiber the medium of choice for premises network applications.

BUCK is market manager-premises, Corning, Inc. For more information, contact him at Buckpd@corning.com.